1. Field of the Invention
The present invention relates to a piezoelectric transformer element made of a ceramic material and, more particularly, to a compact, low-profile piezoelectric transformer element of which down sizing and high reliability are required and which generates a high voltage, and a method of manufacturing the same.
2. Description of the Prior Art
Conventionally, in a backlight inverter for a liquid crystal display, an inverter for lighting a fluorescent tube, a high-voltage power supply circuit for a copying machine, and the like, a winding electromagnetic transformer is used as a high-voltage generating element. A piezoelectric transformer has attracted an attention due to requirements such as reduction in the generated electromagnetic noise, power consumption, height, and the like. FIG. 1 shows the structure of a conventional symmetric Rosen tertiary stacked piezoelectric transformer. In this symmetric Rosen tertiary stacked piezoelectric transformer, of a rectangular ceramic piezoelectric plate 1 of a stacked-layer structure having electrodes formed on its surface, the two end portions of the piezoelectric plate 1 in the direction of length form driving portions 21 serving as the input portions of the piezoelectric transformer. In the driving portions 21, planar input electrodes 2a and 2b, and 3a and 3b, are formed on the upper and lower surfaces, respectively, of the piezoelectric plate 1. These two end portions are polarized in the direction of thickness. Electrodes 4a and 4b, and 5a and 5b, electrically connect the input electrodes 2a and 2b, and 3a and 3b, to internal electrodes (not shown) formed among the stacked layers of the piezoelectric plate 1. The central portion of the piezoelectric plate 1 forms a power generating portion 22 serving as the output portion. In the power generating portion 22, output electrodes 6a and 6b are formed on the upper and lower surfaces, respectively, of the piezoelectric plate 1. This central portion is polarized in the direction of length of the piezoelectric element.
The operation of this conventional piezoelectric transformer element will be described with reference to FIGS. 2A to 2C. FIG. 2A is a schematic sectional view of the piezoelectric transformer, FIG. 2B is a graph showing the distribution of displacement of the piezoelectric transformer obtained when it vibrates in the 3/2-wavelength resonance mode in the direction of length, and FIG. 2C is a graph showing the distribution of stress obtained at this time and the types of stress. When a voltage is applied from external terminals 7a and 7b shown in FIG. 1 to the input electrodes 2a and 2b, and 3a and 3b, of the driving portions 21, an electric field is applied to the driving portions 21 in the direction of polarization. As shown in FIG. 2B, longitudinal vibration in the direction of length is excited by the inverse piezoelectric effect in which the element is displaced in a direction perpendicular to the direction of polarization, and the entire piezoelectric element vibrates. In the power generating portion 22, as shown in FIG. 2C, mechanical strain occurs in the direction of polarization. Then, due to the piezoelectric positive effect in which a potential difference occurs in the direction of polarization, a voltage having the same frequency as that of the input voltage is output from the output electrodes 6a and 6b to external terminals 8a and 8b. If the driving frequency is set to be equal to the resonance frequency of the piezoelectric plate 1, a very high output voltage can be obtained. The output that can be obtained from the piezoelectric transformer element is proportional to the power of mechanical vibration, and is proportional to the sectional area of the piezoelectric plate 1 in terms of structure.
Accordingly, in order to obtain a large output from a piezoelectric transformer element, the element size may be increased or the vibration speed of the piezoelectric transformer element may be increased. To increase the element size is not practical because of the requirement for down-sizing of the equipment on which the power supply is mounted. Therefore, an increase in output of the piezoelectric transformer is achieved by increasing the vibration speed. When, however, the vibration speed is increased, it may exceed the mechanical strength of the element. Then, a desired output cannot be obtained but the element fractures. To increase the element strength while avoiding element breakdown, a fine powder is used as the material to form the piezoelectric ceramic material, and the particle size of the ceramic material after calcination is decreased to make the ceramic material dense. With this technique of increasing the element strength by using a fine powder, however, the finer the powder, the more difficult it becomes to handle. In the green sheet process, powder dispersion is not easy, and the pressing density cannot be stabilized, accordingly making it difficult to sufficiently increase the element strength. When the ceramic particle size of the driving portions or power generating portion decreases, the same piezoelectric characteristics as those conventionally obtained cannot be obtained with the same application voltage as that conventionally applied. In order not to degrade the transformer efficiency, the design must be changed by reconsidering the polarization conditions.
It is proposed to employ a technique for increasing the element strength by improving the structure of the ceramic material that forms the element. For example, Japanese Unexamined Patent Publication Nos. 8-107241 and 2-100306 disclose a stacked ceramic component in which, in order to eliminate a step which is generated by the thickness of the internal electrode and which produces cracking or separation in the stacked ceramic component such as a piezoelectric transformer or ceramic capacitor, spacers each having a thickness almost equal to that of the internal electrode are formed at portions excluding the internal electrodes. More specifically, FIGS. 3A to 3E are exploded perspective views of a piezoelectric transformer element disclosed in Japanese Unexamined Patent Publication No. 8-107241. Spacers 112a to 112e are formed, together with internal electrodes 110a to 110e, at different planar positions of ceramic sheets 111a to 111e to be stacked, in order to prevent cracking and separation during pressing. Similarly, In Japanese Unexamined Patent Publication No. 2-100306 shown in FIGS. 4A and 4B, an internal electrode 210 is formed on one ceramic green sheet 211 that partly constitutes a stacked electronic component. A spacer 212 is formed in the other ceramic green sheet 211 which is to be stacked on the one ceramic green sheet 211, and surrounds the internal electrode 210 of the one ceramic green sheet 211. The spacer 212 prevents cracking and separation in pressing.
With the techniques described in these references, however, a spacer is merely formed at that portion of a ceramic member to be stacked, which excludes an internal electrode. Although these techniques are effective to prevent cracking and separation resulting from the stress generated during pressing or sintering, it is difficult to eliminate element breakdown which results from stress concentration that occurs while the piezoelectric transformer element is driven. In particular, in the symmetric Rosen tertiary piezoelectric transformer as shown in FIG. 1, since the point of concentration of tensile stress is present on a region at the center in the direction of length of the element where the output electrodes are to be formed, the spacer described above does not effectively function against this tensile stress, and in most cases the element fractures at the output electrodes and near them.